This invention relates to temperature compensation in electronic devices, especially in devices including integrated circuits.
Background of the Invention
As described later, the solution according to the present invention is suitable for various different circuit applications in which the effect of temperature fluctuations must be compensated for. One possible object of application is a function generator which can be used in a radio transceiver, for example. A function generator is thus used as an example in the following description.
The main problem of an exponential (or logarithmic) function generator is its temperature stability. A conventional way of compensating for the voltage or current changes caused by temperature fluctuations is to use resistors proportional to the absolute temperature value of the environment, so-called PTAT resistors. This type of resistor is commercially available as a discrete component from different manufacturers.
The use of these discrete components in association with integrated circuits has some drawbacks, however, which are described briefly in the following.
Firstly, such an external resistor increases the costs of the circuit: a discrete PTAT resistor is an expensive component as such, and since its mounting on the circuit board requires time, this also means additional cost. The costs are further increased by the fact that material management becomes more complicated as the external components must be available in stock. Secondly, an external PTAT resistor decreases the reliability of the circuit, i.e. the external resistor shortens the MTTF value (Mean Time To Failure) of the circuit. Thirdly, an external PTAT resistor makes the associated circuit more sensitive to interferences.
Moreover, implementing the temperature compensation with high accuracy is difficult and requires a complicated circuit. This is because the temperature of an external PTAT resistor can be different from that of the chip containing the circuit whose temperature behaviour the resistor controls. Therefore, accurate compensation requires that a temperature measurement be introduced into the circuitry.
The above drawbacks can at least partially be eliminated by integrating the PTAT resistors into the chips. However, this kind of integration requires additional process steps which are non-standard and available only from a small amount of vendors. Thus, such a solution involves high costs which in turn hinder application of this integration to low cost or consumer products, for example. Further, the additional process steps used are vendor-specific. This means that if the same circuit is later ordered from another vendor, certain process steps must be done again. The more complicated the manufacturing process is, the more the vendor-specific processes differ from each other.
The purpose of the invention is to diminish the above-mentioned drawbacks and to create a solution by means of which it is possible to perform the temperature compensation in an integrated circuit device without having to resort to costly manufacturing processes.
This goal can be attained by using the solution defined in the independent patent claims.
The basic idea of the invention is to replace a PTAT resistor used for temperature compensation by a series connection of an ordinary resistor and a resistive element with non-linear temperature characteristics, preferably a (MOS)FET. Owing to this, the temperature compensation can easily be integrated into a chip using standard manufacturing processes. Thus, according to the invention a temperature dependent resistive element is simulated by a series connection of an ordinary resistor and, a resistive element with non-linear temperature characteristics. (xe2x80x9cOrdinaryxe2x80x9d here refers to an element with essentially constant resistance regardless of the temperature value.)
The invention offers an inexpensive solution for temperature compensation in electronic devices. This is because neither the special IC manufacturing nor the external components mentioned above are needed. Consequently, the solution according to the invention is particularly advantageous in high volume/low cost applications (i.e. for example in end-user applications, such as mobile phones).